The invention relates to a method for picking up rapidly moving filaments or threads. The invention is also directed to thread catching devices which are operated by compressed air. After being picked up by the thread catching devices, the filaments are cut and then carried through a propulsion nozzle to a catch site.
Thread catching devices of this type are especially useful where, for example, threads moving in manual operation from a spinning device to a winding bobbin must be picked up and temporarily carried to a catch site when the bobbin must be changed. During the bobbin exchange the thread runs to the catch site so that the spinning process need not be interrupted. Following bobbin exchange the moving threads are taken up by the new winding bobbin.
Known thread catching devices work according to the ejector principle, with compressed air escaping from one or several air nozzles, called propulsion nozzles or jets. The compressed air reaches a propulsion tube and induces a suction effect at the entry to the tube and a high air velocity inside the tube, which results in propelling the threads through the tube. The threads must be intercepted at the entry side in order to reach the propulsion tube where they are transported by the effect of the highest attainable thread tension, that is, by the highest possible propulsion force.
Both a high suction force and high propulsion force are required for the proper operation of such a thread catching device. However, practice has shown that these two forces cannot be brought to their maximum effect independently of each other. Thus, if an ejector nozzle is constructed for high suction, then its propulsion effect is low, and vice versa.
Thread catching devices with switchable nozzles are already known which can be set for high suction effect during filament pickup and thereafter for high propulsion effect. However, such devices have the drawback of carrying the thread in almost tensionless condition during the suction period, thereby creating a time lag before the nozzle can be switched to high propulsion. Leaving the threads almost tensionless creates a great danger of interruption of the thread feed between the thread supply and the thread catching device.
Another known method uses small projectiles for facilitating the smooth pickup of filaments by the thread catching device, but this method is cumbersome because of the necessity of disentanging the projectile from the filaments.
Further, a thread catching device is known which has a lead-in passage in front of the suction end of the main propulsion nozzle. The filaments or threads introduced into the lead-in passage are blown to the suction end of the main propulsion nozzle by means of a supplemental propulsion nozzle and a borehole running co-axially with the main propulsion nozzle. The supplemental propulsion nozzle necessarily makes the device more complicated. Besides, the air stream from the supplemental propulsion nozzle must again be accelerated by the main propulsion nozzle which prevents the attainment of the maximal achievable air velocity in the propulsion tube, and thereby prevents the highest possible thread tension. In addition, the threads enter the propulsion tube perpendicularly to its longidutinal axis and must consequently be re-directed by a right angle. This creates undesirable tension-reducing friction.
It is therefore the objective of the present invention to lead the threads to the thread catching device without auxiliary means and to pass them on immediately with maximal thread tension and velocity.